-LRB- CNN -RRB- -- Enough Afghan opium to supply world demand for two years has effectively gone missing , with the Taliban suspected of stockpiling supplies in a bid to corner the market , the United Nations Office for Drugs and Crime -LRB- UNODC -RRB- has revealed .

Pakistani customs officials destroy contraband narcotics on the border with Afghanistan .

Afghanistan is the world 's leading narcotics supplier . Earlier this month , a U.N. study revealed Afghanistan 's opium production had dropped dramatically this year partly because of new aggressive drug-fighting tactics in the country .

According to the UNODC report , production dipped by 10 percent this year while cultivation fell by 22 percent .

However , a senior U.N. spokesman warned that this positive news should be treated with caution .

`` We figure the world needs around 4,000 tons of opium a year for licit and illicit purposes , '' Walter Kemp of the UNODC told CNN . Has enough empasis been placed on drug trafficking ?

`` But this year around 6,900 tons was produced , with 7,700 tons delivered last year and more than 8,500 the year before that . Map showing where Afghan opium is going ''

`` So if the world only needs around 4,000 tons of opium and a further 1,000 is seized , where is the rest of it going ? ''

According to Kemp , world demand for opium remains stable yet prices are not crashing , which suggests a large amount of opium is being withheld from the market .

`` Our guess is that around 12,000 tons of opium has been stockpiled somewhere -- not all in one place but in and around Afghanistan , '' he added . `` So while production might be coming down -- mostly because of market reasons -- there 's still a lot of product around to satisfy demand for about two years . ''

It is unclear exactly who is responsible for this but there 's growing evidence , according to the U.N. , that the Taliban are becoming increasingly involved in the industry and could be sitting on huge stockpiles of opium to use as credit for financing their activities .

`` Farmers will be keeping small amounts back as credit for things such as a dowry or buying livestock , '' said Kemp . `` But they wo n't have the means to store supplies in the kind of quantities we 're talking about here .

`` It 's probably in the hands of people with the ability to store it underground and to keep people away from it through corruption or force . ''

Hakan Demirbuken , a research expert on the U.N. 's Afghan Opium Trade Program , said Taliban involvement in the drugs trade is not limited to taxing Afghan opium farmers and traders in return for their `` protection . ''

He told CNN : `` Last year we estimated that Afghan poppy farmers earned around $ 730 million , while traders who take the product on to the border earned around $ 3.4 billion . ''

`` From this lucrative business the Taliban took around $ 125 million in tax .

`` But according to U.N. figures they need around $ 800 million per year for their operational needs . ''

However , most of the trade is controlled by organized criminal groups from outside Afghanistan . Therefore Demirbuken believes groups such as the Taliban and al Qaeda will be forging links with criminal gangs in order to become more involved in the production and trafficking stages .

In addition to the increased revenue greater involvement would provide , he said groups such as al Qaeda `` will have noted the destabilizing effect this industry -- and the sums of money it generates -- can have on more vulnerable countries with weak governments . ''

In October last year , the United States told NATO members that the drug trade was a threat to coalition troops because there was a direct connection between it and Taliban insurgents in Afghanistan .

`` There is what we call a nexus of insurgency . There 's a very broad range of militant groups that are combined with the criminality , with the narco-trafficking system , with corruption , that form a threat and a challenge to the future of that great country , '' then-U.S. commander in Afghanistan Gen. David McKiernan told reporters .

As a result NATO combat forces have now been actively attacking militants , drug laboratories and buildings connected to insurgents with ties to drug lords for the first time since the start of the Afghan conflict in 2001 .

Meanwhile , international law enforcement organization , Interpol , believes there has been a change of tactic involving Afghanistan 's opium , with much more of it being turned into heroin within the country and stockpiled or couriered out , primarily through Iran .

Historically Afghanistan has been responsible only for cultivating raw opium , with the conversion into a final product taking place across the border in Pakistan or in Iran and Turkey , according to the UNODC .

Producing heroin in Afghanistan makes it easier to conceal and transport than the bulkier raw opium .

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Recent study revealed Afghan opium production has dropped dramatically

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U.N. says this should be treated with caution as country has been over-producing

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Taliban suspected of stockpiling large amounts of opium `` as credit ''

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U.S. has warned of growing link between Taliban and the drug trade